The present invention relates to an improved process for the production of polysiloxane-based polymeric compositions useful in the manufacture of biocompatible medical devices. More particularly, the present invention relates to an improved process for the production of polysiloxane-based polymeric compositions that eliminates difficulties experienced in preparing polysiloxane prepolymers to completion and difficulties experienced in the subsequent purification thereof.
Since the 1940""s optical devices in the form of intraocular lens (IOL) implants have been utilized as replacements for diseased or damaged natural ocular lenses. In most cases, an intraocular lens is implanted within an eye at the time of surgically removing the diseased or damaged natural lens, such as for example, in the case of cataracts. For decades, the preferred material for fabricating such intraocular lens implants was poly(methyl methacrylate), which is a rigid, glassy polymer.
Softer, more flexible IOL implants have gained in popularity in more recent years due to their ability to be compressed, folded, rolled or otherwise deformed. Such softer IOL implants may be deformed prior to insertion thereof through an incision in the cornea of an eye. Following insertion of the IOL in an eye, the IOL returns to its original pre-deformed shape due to the memory characteristics of the soft material. Softer, more flexible IOL implants as just described may be implanted into an eye through an incision that is much smaller, i.e., less than 4.0 mm, than that necessary for more rigid IOLs, i.e., 5.5 to 7.0 mm. A larger incision is necessary for more rigid IOL implants because the lens must be inserted through an incision in the cornea slightly larger than the diameter of the inflexible IOL optic portion. Accordingly, more rigid IOL implants have become less popular in the market since larger incisions have been found to be associated with an increased incidence of postoperative complications, such as induced astigmatism.
With recent advances in small-incision cataract surgery, increased emphasis has been placed on developing soft, foldable materials suitable for use in artificial IOL implants. One such suitable class of soft, foldable materials is silicone elastomers fabricated through the polymerization of divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane with polysiloxanes having multiple hydrosilane groups. This silicone elastomer producing polymerization reaction is achieved under thermal conditions using a platinum catalyst. A component added to the described siloxane and polysiloxanes prior to initiation of the polymerization reaction, is a reinforcing agent to enhance the mechanical properties of the silicone elastomer end product so fabricated. Examples of suitable reinforcing agents include a silica filler or an organic reinforcing resin such as polysiloxane with a vinyl functional group.
The prepolymer, divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane used in the polymerization reaction described above, is prepared by reacting a 1,3-bisvinyl tetraalkyldisiloxane, a mixture of octamethyl cyclo-tetrasiloxane and an aromatic group containing cyclosiloxane, especially octaphenylcyclo-tetrasiloxane. Using an amine or a potassium silanoate as a catalyst, the reaction used to prepare the noted prepolymer is carried out at 40-100xc2x0 C. in neat or in an organic solvent. This polymerization reaction only reaches an equilibrium with some cyclics, either those of the original components or those regenerated from the growing polymer, which then remain as side products. The resulting product was purified using a high temperature, high vacuum, thin film evaporator to remove solvent and volatile cyclics. Because of the poor solubility of the aromatic cyclics, incorporating a quantitative amount of the aromatic cyclics into the growing polymer molecule proved difficult. Likewise, due to high melting points, the aromatic cyclics have no vapor pressures and can not be removed using the thin film evaporator. As a result, in most cases, the aromatic cyclics remain as a contaminant in the final silicone elastomer product. The presence of aromatic cyclics as contaminants in the final silicone elastomer product creates the potential for defects and possible failures in products produced therefrom.
Because of the noted shortcomings in the quality of divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane prepolymer using the described known process, there is a need to have an improved process for synthesizing the divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane prepolymer.
Soft, foldable, high refractive index, high elongation, polymeric compositions or silicone elastomers are prepared in accordance with the present invention through the copolymerization of aromatic-substituted polysiloxane prepolymers with one or more polysiloxanes having multiple hydrosilane groups. The improved production process of the present invention eliminates difficulties formerly encountered in preparing polysiloxane prepolymers to completion as well as difficulties formerly encountered in the subsequent purification thereof. Following preparation of the polysiloxane prepolymers using the process of the present invention, the prepolymers are copolymerized with selected hydro-silane-containing polysiloxanes and other comonomers/components to form desirable polymeric compositions useful in the manufacture of biocompatible medical devices such as ophthalmic devices. Such desirable polymeric compositions are transparent, relatively high in strength for durability during surgical manipulation, relatively high in elongation, relatively high in refractive index and particularly well suited for use in the manufacture of ophthalmic devices such as intraocular lens (IOL) implants, contact lenses, keratoprostheses, corneal rings, corneal inlays and the like. Medical devices fabricated from the polymeric compositions or silicone elastomers produced using polysiloxane prepolymers prepared in accordance with the present invention are of improved product quality and reliability.
The process of the present invention is used to prepare divinyl-end capped poly(dialkyl)-co-(diaromatic substituted) siloxane prepolymers having a structure generally represented by Formula 1 below: 
wherein the R1 groups may be the same or different alkyl substituents or aromatic substituents; the R2 groups may be the same or different alkyl substituents; the R3 groups may be the same or different aromatic substituents; and x and y may be the same or different natural numbers so that x/x+y is at least equal to 0.5 and each OSi(R2)2 and each OSi(R3)2 are independently and randomly distributed in the prepolymer molecule.
Accordingly, it is an object of the present invention to provide a process for the production of transparent, biocompatible polymeric compositions having desirable physical characteristics and relatively high refractive indices.
Another object of the present invention is to provide a process for the production of polymeric compositions having relatively high refractive indices and good clarity.
Another object of the present invention is to provide a process for the production of polymeric compositions suitable for use in the manufacture of ophthalmic devices.
Another object of the present invention is to provide a process for the production of polymeric compositions suitable for use in the manufacture of intraocular lens implants.
Still another object of the present invention is to provide a process for the production of polymeric compositions that are economical to produce.
These and other objectives and advantages of the present invention, some of which are specifically described and others that are not, will become apparent from the detailed description and claims that follow.